I have thought about just this problem in the past many times. The difficulty that I have is that since the model's head, or any other part, is probably not flat, the angle and distance that has to travel is subject to a correction based on the overall curvature presented within the plane of the now oval shaped light. This means that the number of quanta within the beam will have different strengths and the problem cannot be solved with a linear equation. That is why I use a light meter, bracket, and take lots of shots of the same thing! Stimulating though, haven't heard "quanta" for a while.
More meds please
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